In order to form a basis for understanding the detailed mechanism of oxygen transport in biological systems, we wish to determine, by single crystal X-ray diffraction techniques, the structures of a number of oxygen-carrying complexes of cobalt and other transition metal complexes. It is presently feasible to determine the geometry and interatomic distances in these model compounds with a high degree of accuracy; this is so, even though most of the work in this field to date has been of fairly low precision (thus its accuracy is hard to assess). We have improved this precision by an order of magnitude in our studies of five different cobalt compounds and we propose to extend our work to the synthesis and structural characterization of various substituted bis (salicylaldehyde)ethylenediimine complexes of cobalt, iron, copper, or other transition metals. Variation of the substituents on the salicylaldehyde ring and the ethylenediamine bridge as well as variation of the axial base will affect both the oxygen-carrying properties of the complexes and their ease of crystallization; for such of these as can be induced to form crystals, we will attempt to explain the variation in structure in terms of electronic effects at the metal and the oxygen molecule. Until a technique is available for determining directly the metal-oxygen geometry in metalloproteins-and none is available now--work such as this on model compounds continues to be important.